Koharu Minagi is a high school student who endured significant family tragedy in childhood when her biological mother and younger brother perished in a car accident. After this loss, she lived inland with her father, Issei Minagi, far from the ocean, cultivating a deep fascination with the sea. Overseeing household responsibilities alone from a young age due to her father's work schedule honed her strong cooking skills. Coping with asthma limited her outdoor activities, leading to proficiency in indoor games.

Her family circumstances shifted when her father remarried, making Hiyori Minagi her stepsister and Hinata Minagi her stepmother. Koharu first met Hiyori at the seaside, where her excitement about seeing the ocean nearly led her to impulsively enter the water. This seaside encounter initiated their relationship, evolving through shared experiences in fly fishing—a hobby Koharu learned from Hiyori despite having no prior exposure to fishing.

Physically, Koharu stands 155 cm tall with cream-blonde hair typically styled in two side ponytails cascading to her waist and reddish-brown eyes. She favors cute outfits, often wearing a school uniform featuring a ribbon and white knee-high socks. During fishing trips, she coordinates matching red boots with Hiyori and their friend Koi Yoshinaga.

Koharu exhibits a cheerful, expressive personality, frequently voicing her thoughts openly with a smile, sometimes perceived as blunt. She demonstrates a tendency toward jealousy and imitation of others, prompting friends to describe her as "troublesome." Despite her outwardly carefree demeanor, she harbors deep insecurities about being disliked and masks sadness or pain behind childish behavior to avoid burdening others. Her approach to relationships is calculating to maintain harmony, though she struggles with deeper connections outside her close circle.

Her abilities include exceptional cooking, which remains her primary household role even after the remarriage, as her stepmother lacks culinary skills. Academically, she excels in English with strong pronunciation but struggles with mathematics. She experiences car sickness and avoids strong boat movements, though she does not get seasick. Her interests extend to cute concepts, including a desire to open a themed fish restaurant.

Key relationships include her stepsister Hiyori, with whom she forms a strong bond through fishing and daily life, often expressing pride in their sisterhood. Her relationship with stepmother Hinata evolves from initial distance to closeness through shared activities like shopping, eventually leading Koharu to comfortably address her as "Mom." Friendships with Koi Yoshinaga, a classmate and fishing companion, and Miyabi Tsuchiya, a middle school friend who understands her true self, are significant, though Koharu typically remains peripheral in deeper group conflicts.

Across her development, Koharu gradually becomes more honest about her emotions. Early tendencies to mask her feelings give way to occasional assertiveness in expressing desires, strengthening family bonds. Conflicts arise, such as Hiyori urging her to mature emotionally, causing Koharu distress, but reconciliation follows through shared experiences. Her journey involves balancing newfound family happiness with lingering grief for her deceased mother and brother, ultimately fostering the courage to embrace vulnerability with her new family.